1. Field of the Invention
This invention relates generally to methods of dimensionally-stabilizing fluid-like elastomeric polymers. More specifically, the invented methods relate to stabilizing composite polymer-ceramic materials for use as solid-state battery electrolytes/separator components, wherein the resulting composite material possesses the high conductivity of a polymer electrolyte and the enhanced durability of a ceramic material. This invention relates to a new molecular composite material for use in all-solid-construction reactive metal batteries. The invented materials are primarily designed for use in either reserve or primary reactive metal/water batteries.
2. Description of Background Art
A battery typically comprises one or more electrochemical cells connected in series, parallel, or both, depending on desired output voltage and capacity. Each cell principally comprises an anode, a cathode, and an electrolyte. The electrolyte serves as the ionic conductor and provides the medium for the transfer of ions inside the cell between the anode and the cathode, and typically comprises liquid, solid, or gel materials. Some batteries, commonly called “primary batteries,” are intended for a single use, and, once discharged, are discarded. Other batteries, commonly called “secondary or rechargeable” batteries, are designed to be recharged, after discharge, essentially to their original condition. During discharge, ions from the anode pass through the liquid electrolyte to the electrochemically-active material of the cathode where the ions are taken up with the simultaneous release of electrical energy. During charging, the flow of ions is reversed, so that ions pass from the electrochemically-active cathode material through the electrolyte and are plated back onto the anode.
Solid polymer electrolytes are useful in numerous applications, such as solid-state batteries, supercapacitors, fuel cells, sensors, electrochromic devices and the like. Solid polymer electrolytes have been proposed in the past for use in such equipment, in place of liquid electrolytes, because they combine in one material the function of electrolyte, separator, and binder for the electrode materials, thereby reducing the complexity of the ultimate structure. The advantages inherent in the use of a solid polymer electrolyte (SPE) are the elimination of possible liquid leakage and prevention of dangerous increases in pressure sometimes occurring when volatile liquid electrolytes are present. Further, such SPEs can be fabricated as thin films, which permit space-efficient batteries to be designed. Also, flexible solid polymer electrolytes can be fabricated, which allow for volume changes in the electrochemical cell without physical degradation of the interfacial contacts.
Significant improvement of solid polymer electrolyte materials, over the materials available in the past, is needed in order for all-solid-state batteries to be commercially useful. New SPE materials must be excellent conductors of ions at ambient temperatures, as high internal resistance is the most pressing problem in SPE batteries today. Current organic SPE systems are poor ion conductors at ambient temperatures and the most common strategy employed to combat this problem is to use small organic molecules as additives. See, for example, Abraham, et al., U.S. Pat. No. 5,219,679. While this strategy does result in increased ion transport, current commercial additives suffer from numerous problems such as flammability, toxicity, and a lack of oxidative stability. However, phosphazenes exhibit many favorable properties including high ion conductivity, oxidative stability, non-flammability and non-toxicity. Recent research has focused on improving the mechanical properties and ion transport abilities of polymeric phosphazenes.
Additional problems with SPEs are low conductivity, low dimensional stability, and the manner in which mobile cations are introduced into the matrix. Current methods for addressing these problems are through the use of fillers and the introduction of ions as low lattice energy salts (e.g. triflates). See, for example, Gao, U.S. Pat. No. 6,020,087.
A number of SPEs have been suggested for use in the prior art such as thin films formed by complexation between lithium salt and linear polyethers. See, for example, Narang, et al., U.S. Pat. No. 5,061,581.
Attempts have been made to improve the ionic conductivity of polymer electrolytes by a selection of new polymeric materials such as cation-conductive phosphazene and siloxane polymers. Other suggestions include the use of the addition of plasticizers to polymer electrolytes to form a gel electrolyte. See, for example, Sun, U.S. Pat. No. 5,609,974. While this procedure does improve ambient temperature conductivity, this is done at the expense of mechanical properties.
Attempts have also been made to improve the dimensional stability of phosphazene films (described by Ferrar et al., Polyphosphazene Molecular Composites, 20, 258–267 (1994)). Ferrar produced an anti-static film with improved dimensional stability while maintaining transparency and negative adhesion. Ferrar was not concerned with ionic conductivity, and said anti-static film did not exhibit sufficient ionic conductivity to serve as a commercially useful electrolyte.
Attempts to obtain polymer electrolytes with high conductivity at room temperature have lead to the study of polymers that are highly flexible and have largely amorphous morphology, because the prevailing theory is that ionic conductivity is facilitated by the large-scale segmental motion of the polymer and that ionic conductivity principally occurs in the amorphous regions of the polymer electrolyte. Crystallinity is understood to restrict polymer segmental motion and significantly reduce conductivity. Consequently, the ionic conductivity of complexes of alkali metal salts with poly(ethyleneoxide) has been observed. Li salt complexes of polymers such as poly[bis(methoxyethoxyethoxy)phosphazene] (MEEP) and poly(ethoxyethoxy-ethoxy-vinyl ether) (described by Guglielmi et al., Appl. Organometal. Chem. 13, 339–351 (1999)), prepared on the basis of these principles, have shown room temperature conductivities of around 10−5 S/cm. While the ionic conductivities of such polymers at ambient temperatures have fallen within acceptable limits for battery applications, they suffer from physical drawbacks, making them inappropriate for use as electrolytes. MEEP, for example, suffers from very low dimensional stability that prevents its extensive use in battery construction technology. Specifically, MEEP is in the visco-elastic flow regime at ambient temperature, and can therefore flow like a viscous liquid without retaining its form when subjected to an external force.
Allcock et al. (U.S. Pat. No. 5,414,025, issued May 9, 1999) disclose a method of crosslinking of polymeric electrolytes, wherein UV radiation is used to increase the structural integrity of polyorganophosphazenes, including MEEP, by inducing C—H bond cleavage to form C—C bond crosslinks. The involves forming a film of MEEP on glass, irradiating the film at between 220 and 400 nm., and then extracting the swollen gels in tetrahydrofuran. The Allcock et al. methods include adding a photoinitiator to increase the amount of crosslinking. While Allcock et al. teach technology that purposely produces substantially-crosslinked polymer film wherein the crosslinking is present throughout the entire polymer electrolyte. The inventors of the present invention, as well as others in the field, have shown that such crosslinking, which may be called “homogeneous” crosslinking, substantially inhibits lithium ion transport.
In summary, no commercially-useful SPE is known in the prior art. In other words, no SPE is known in the prior art that is a thin film that possesses good mechanical properties, including processability, dimensional stability, and durability, while also possessing appropriate ionic conductivity in the range of 10−4 S/cm at ambient temperatures and appropriate electrochemical stability.
Therefore, there is still a need for a dimensionally-stable, durable polymer electrolyte for use in several different classes of reactive metal batteries, such as Li/water primary or reserve batteries. There is still a need for a stable, durable electrolyte that exhibits high ionic conductivity, and has good processability by virtue of being less adherent, and intractable than previous polymer-gel electrolyte materials. The present invention addresses these needs.